4 electron transport complexes have been identified and 3 of them are proton pumps--complex I, III, IV. complex I oxidizes NADH, and passes on electrons which NADH carries to complex III, IV, then to the terminal acceptor O2. for every electron passed, 4 protons are passed by complex I and III, and only 2 are passed by complex IV. complex II is not a transmembrane complex (thus not a proton pump) and it oxidize FADH2. thus electrons donated from FADH2 only pass through complex III and IV. therefore, for every electron donated by NADH, 10 protons are pumped across membrane, and for every electron donated by FADH2 only 6 protons are pumped.

As proton gradient builds up across membrane, protons pass through ATP synthase to generate ATP's. roughly 4 protons are needed for 1 molecule of ATP. Therefore, for every NADH molecule, 2.5 ATP molecules are generated, and for every FADH2 molecule, 1.5 ATP molecules are generated.

so every reaction of succinate to fumarate produces 1.5 ATPs and every reaction of malate to oxaloacetate produces 2.5 ATPs.